In photolithography using chemically amplified resists, the area of photoresist film exposed to incident radiation undergoes a chemical transformation. For most resists systems designed for traditional aqueous base development, this transformation results in a significant increase in polarity. For these systems, traditionally termed positive tone resist systems, the exposed region is removed during development with an aqueous base. Aqueous solutions of tetramethyl ammonium hydroxide (TMAH) are most commonly used as the photoresist developer. In order for development in aqueous base to be effective, the base must deprotonate a certain number of Bronsted acid groups to allow the resist to become soluble. The amount of deprotonation required for photoresist solubility is known as the critical ionization level, and has been well described in the literature (see G. Willson, et al. J. Vac. Sci. Technol. B 20(2) March/April 2002, 537-543).
Feature sizes on microelectronic devices continue to shrink as these devices become smaller, faster and more powerful. The lithographic development of these smaller feature sizes in aqueous base can become problematic due to image collapse caused by the capillary forces and surface tension of water. Current approaches to solving this problem include IPA vapor drying and bilayer resist technology. The prevention of image collapse in supercritical CO2 dryers for MEM's and other applications is based on the absence of surface tension in supercritical CO2 to avoid the capillary forces that cause image collapse. Each of these potential solutions for image collapse requires additional steps in the lithographic process leading to higher cost and decreased device yield.
Non-smooth edges on developed features become problematic as feature sizes get smaller (Semiconductor International, February 2005; p. 44). The roughness of a single edge is known as Line Edge Roughness (LER) and the roughness of a feature defined by two edges is known as Line Width Roughness (LWR). Current approaches to minimizing LER/LWR include modification of the photoresist or etch chemistry, back anti-reflection coating (BARC), or use of a hardmask. All of these approaches generally result in decreased imaging and/or etch performance of the resist, or require additional process steps that increase cost and decrease device yield. Densified CO2 has the ability to penetrate and swell certain amorphous polymers. This facilitates delivery of chemistry into the swollen polymer, and can smooth out surface features. Under proper conditions, these properties can be used in lithography to smooth surfaces without affecting the critical dimensions of a feature.
Commercially available photoresists used for 248-nm lithography, 193-nm lithography, e-beam lithography, and those being developed for EUV-based lithography are not soluble in liquid or supercritical carbon dioxide in the exposed or unexposed state making CO2-based development extremely challenging. Furthermore, Bronsted bases such as TMAH are neutralized in supercritical carbon dioxide which acts as a weak acid. As such, a pH above 7 is not readily accessible in CO2 based systems. Under these conditions, the minimum level of ionization needed to dissolve exposed positive tone photoresist is not achievable.
In carbon dioxide solvent systems, low-polarity polymer species, specifically fluorinated polymers and siloxane-containing polymers are more soluble than polar polymers. This provides an obvious pathway for negative tone image development, as seen in U.S. Pat. No. 5,665,527 to Allen and U.S. Pat. No. 6,379, 874 to Ober. However, for Allen and Ober, image transfer in the negative tone using dense CO2 utilizes non-commercial and in some cases impractical fluorinated or siloxane-containing polymers that are not proven resist systems and are unlikely to be adopted by the microelectronics industry.
Hence there is a need for carbon dioxide based development systems that are compatible with traditional 248-nm, 193-nm, 157-nm, e-beam and EUV resists and leverage the physical properties of CO2 as a processing fluid to give reduced image collapse and decreased LER/LWR.